The invention relates to phased array antennas and more particularly to such antennas which employ binary digital phase shifters between the transmitter/receiver of the system and the radiator elements of the array. Phased array antennas provide for steering of the antenna beam by controlling the relative phases of the energy applied to the radiating elements of the array. A simple linear array will produce a broadside beam having its direction of maximum radiation, or boresight axis, normal to the direction of the array if the radiating elements all have energy of the same phase applied thereto. If the phases of the energy at the antenna elements progress in a linear manner along the array, an off-broadside beam will result, the direction of which will depend on the phase difference between each adjacent element relative to the spacing thereof. Thus by programming the phase shifters in a predetermined pattern, the beam can be made to scan back and forth around the broadside position. Prior art phased arrays have usually used analog phase shifters which can be adjusted to any desired phase shift in response to a control signal to provide a desired beam scanning pattern. Such continuously variable phase shifters are complex and expensive and often have rather low reliability. Failure of one or more of these phase shifters can seriously degrade the performance of an entire antenna array. A simpler phase shifter has been devised by connecting a plurality of elementary binary digital phase shifters in series. These elementary phase shifters are capable of assuming two different phase shifts, one of which is zero. If several of these elementary binary phase shifters are connected in series, the total phase shift will be the sum of the phase shifts of each individual binary phase shifter. If a series of such elementary phase shifters each have a different "on" phase shift and each phase shifter has twice the "on" phase shift of the preceding adjacent elementary phase shifter, different combinations of the elementary shifters of the series can be switched "on" or "off" to achieve 2.sup.n different total phase shifts, with the incremental phase shift equal to that of the "on" phase shift of the first element. For example, suppose three binary phase shifters are connected in series and the first shifter can be switched between 0.degree. and 45.degree., the second between 0.degree. and 90.degree., and the third between 0.degree. and 180.degree.. Such a three stage binary digital phase shifter can be switched in increments of 45.degree. from 0.degree. to 315.degree. in eight steps. For many phased array antenna applications this accuracy of phasing of the antenna radiators is sufficient to produce a satisfactory radiation pattern.
In order to reduce the amplitude of antenna pattern sidelobes, phase array antennas often have the power distribution to the array tapered so that the center elements get maximum energy from the transmitter and the side elements are tapered to a minimum energy at the two elements at opposite ends of the array. In such tapered arrays, the low energy end elements contribute relatively little to the far-field radiation pattern and hence the phase accuracy of these elements can be considerably less than that of the high energy center elements. Thus the number of stages and hence the phase accuracy of the binary phase shifters can decrease on either side of the center radiators. Such a tapered phased array with digital phase shifters comprises one aspect of the present invention.
Further, the present invention provides for greater reliability in the operation of phased antenna arrays by providing redundant circuitry for the digital binary phase shifters thereof so that the system can continue operation even though up to 50% of the elementary binary phase shifters stages may become inoperative or stuck at a fixed phase.
Other aspects of the invention comprise a method of optimizing the performance of the antenna arrays disclosed herein when one or more of the phase shifters connected to an antenna radiator becomes inoperative.